Due to the increase in application of foils, fibers and yarns under dynamic loading, characterizing the behavior of materials at high rates of strain becomes important. It is known that adequate experimental techniques, such as the Split Hopkinson Bar technique, have been established for measuring the dynamic behavior of materials at high strain rates. However, due to the low mechanical impedance and small size of the polymer or composite specimens, the conventional Split Hopkinson Bar technique would encounter serious problems, such as unacceptable high noise-to-signal ratios, undistinguishable transmitted signal, short loading time and achieving the maximum strain....[ Read more ]

Due to the increase in application of foils, fibers and yarns under dynamic loading, characterizing the behavior of materials at high rates of strain becomes important. It is known that adequate experimental techniques, such as the Split Hopkinson Bar technique, have been established for measuring the dynamic behavior of materials at high strain rates. However, due to the low mechanical impedance and small size of the polymer or composite specimens, the conventional Split Hopkinson Bar technique would encounter serious problems, such as unacceptable high noise-to-signal ratios, undistinguishable transmitted signal, short loading time and achieving the maximum strain.

In this research project, a mini-Split Hopkinson Tensile Bar (mini-SHTB) system has been developed to measure the constitutive relation of micro-scaled material specimens under high strain rates. The system employs polymeric bars of small diameter to achieve a closer impedance match with the specimens. This match ensures a lower noise-to-signal ratio in the transmitted signals and hence allows correct interpretation of the transmitted strain profiles. The incident wave of the mini-SHTB is generated by the action of a pendulum. The magnitude and duration of the pulse are controlled by the use of a pulse shaper. The specimen under test is gripped by a special fixture to avoid slippage occurring during rapid loading.

Since the bar material is visco-elastic in nature, the one dimensional elastic wave theory is no longer applicable. Linear visco-elastic models are used to correct the dispersion and attenuation of waves propagating in visco-elastic bars. To correlate the strain signals at the measurement points to the bar-specimen interface and to reconstruct the stress and particle velocity at the ends of the specimen, the characteristic method is applied. This visco-elastic analysis method is verified with a simple experiment. Good agreement is demonstrated between the analytical prediction and experimental measurement.

A series of experiments were carried out on different types and forms of material, such as aluminum foil, cellulose nitrate foil, badminton racket string and so forth, to verify the applicability of the apparatus. Strain rates in the order of 10² were attained under this mini-SHTB. The results confirm that the tensile stress-strain behavior of small-sized specimens with low mechanical impedance under high strain rate can be determined effectively and efficiently using this technique.